1. Field of the Invention
The invention relates to an electric power steering.
2. Description of Related Art
In an existing art, an electric power steering (EPS) controls an electric motor so that steering assist force has an assist characteristic based on the running condition of a vehicle.
For example, a method of controlling the electric motor includes field-weakening control. Field-weakening control is executed only when the vehicle speed is lower than a predetermined value, and is not executed when the vehicle speed is higher than or equal to the predetermined value (for example, see Japanese Patent Application Publication No. 2008-068769 (JP-A-2008-068769)). By so doing, the linearity of the motor characteristic is maintained against steering during running of the vehicle, and motor output according to a command value may be obtained. Therefore, a steering feeling during running improves. On the other hand, field-weakening control works when the vehicle is stopped or the vehicle is running at a low speed and, as a result, required torque may be obtained, so it is possible to ensure appropriate assist force.
In another related art, two electric motors having different output characteristics are arranged at different portions of a steering system from a steering wheel of a vehicle to steered wheels, and the electric motors each are individually controlled on the basis of the running condition of the vehicle to thereby, for example, assist in steering only with the low-power electric motor during running at a high speed and assist in steering only with the high-power electric motor during running at a relatively low speed (for example, see Japanese Patent Application Publication No. 2005-247214 (JP-A-2005-247214)). By so doing, it is possible to easily control steering assist force on the basis of the running condition of the vehicle.
However, as in the case of the above related examples, when the output characteristic of a single electric motor is frequently varied on the basis of the running condition of a vehicle, components may be degraded because of heat and vibrations of the motor due to a steep torque variation to thereby lead to a decrease in the reliability of the system, and, in some cases, it is conceivable that a steering feeling deteriorates. In addition, because a single electric motor is controlled, there is a problem that, when there occurs poor energization in any one of the phases (any one of U, V and W phases) of the electric motor due to a break in a power supply line, a contact failure of a driving circuit, or the like, the system should be immediately stopped.
On the other hand, when two electric motors having different output characteristics are arranged at different portions of a steering system from a steering wheel of a vehicle to steered wheels, the portions at which the two electric motors are installed are different, so a steering feeling may include a feeling of strangeness at the time of changing between the two electric motors. Other than the above, space for installing the two electric motors is required, so the flexibility of installation problematically reduces. Thus, there is still room for improvement in the above points.